1. Field of the Invention
The present invention is directed to a roll machine for manufacturing or treating a material web. More specifically, the present invention is directed to a roll machine with a number of rolls that are disposed in a stack and form a number of nips therebetween for treating or manufacturing a material web, where at least two of the rolls are driven. The present invention also relates to a process for operating such a roll machine.
2. Discussion of the Background Information
DE 295 18 424 U1 discloses a roll machine, particularly a calender. The drive devices of the rolls control the individual rolls to match the speed of the web traveling through before the nips are closed. This matching of speed prevents a speed differential when the nips are closed on the web that might otherwise stress or tear the web.
Such a roll machine can be used both as a calender and as a calender stack, preferably for the treatment or manufacture of a paper web. The paper web is subjected to a certain pressure in the nips between adjacent rolls for several purposes, including evening out irregularities in the surface of the paper web, compressing the paper web, and providing the surface of the paper web with a desired smoothness and/or a desired gloss.
Other material webs, for example made of plastic or aluminum, can also be treated in a similar manner in calenders or calender stacks.
In any case, a drawback of this prior art roll machine is that, after a certain operating time, lateral stripes of varying gloss form on the material web. Material webs with such lateral strips, known in the industry as "barring," cannot be used and are discarded. The corresponding roll must also be replaced or refinished.
The cause of barring has not yet been conclusively identified. One possible cause is initial defects, such as thickness fluctuations in the material web resulting from a periodically fluctuating headbox, cause the rolls and/or their jackets to oscillate at their natural frequency. This can consequently form markings in the surface of one or more rolls, and gradually cause a roll to become polygonal rather than cylindrical. This leads to a corresponding repercussion in the material web so that the polygon shape becomes even more pronounced over time. The lateral strips then become visible after a certain point.
In addition, a polygonal roll produces oscillations that propagate through the entire roll machine, causing malfunctions in other nips. Such oscillations are investigated, for example, in "Barringbildung am Glattkalander einer Papiermaschine" (Barring Formation in the Calender Stack of a Paper Making Machine) by M. Hermanski, Das Papier, Vol. 9, 1995, pp. 581-590. A solution of using more wear resistant surfaces of the roll covers of the soft rolls has been proposed. In "Vermeidung von Glattwerkmarkierungen im Papier mit Escher Wyss Nipco-Walzen" (Preventing Calender Stack Markings in Paper with Escher Wyss Nipco Rolls), Technische Rundschau Sulzer February 1977, pp. 83-89, the authors ascribe the formation of barring to oscillations of the roll machine and propose employing a particularly well-damped roll, namely the Nipco roll, to reduce oscillations.
Nevertheless, barring can be observed even with more wear resistant roll covers and with the use of deflection adjustment rolls with hydrostatically functioning support shoes.